1. Field of the Invention
The invention relates to a projection objective for imaging a pattern arranged in an object surface of the projection objective into an image field arranged in the image surface of the projection objective with a demagnifying imaging scale, to a microlithography projection exposure apparatus having such a projection objective, and to a reflective mask suitable for use in such a projection exposure apparatus. A preferred field of application of the invention is so-called “maskless lithography”.
2. Description of the Related Prior Art
Photolithographic projection objectives with a demagnifying imaging scale (reduction objectives) have been used for several decades for the photolithographic fabrication of semiconductor components and other finely patterned devices. They serve for projecting a pattern, usually provided on a mask (photomask or reticle) onto an article coated with a light-sensitive layer with very high resolution on a demagnifying scale. Most conventional masks carry a fixedly predefined pattern corresponding e.g. to a specific functional layer of a semiconductor component. Such masks with fixedly pre-defined patterns encompass various types of masks, for example binary masks, diverse phase shift masks or hybrid masks. They may be formed as a transmission mask or reflection mask. Masks are also referred to hereinafter as reticles. Demagnifying imaging scales β of 1:4 or 1:5 have become customary in the meantime in conventional projection systems.
Conventional projection systems are designed to image a planar mask onto a planar image field. Accordingly, measures for correcting the image field curvature (Petzval correction) are provided in the projection objectives. For projection lithography on curved substrates, the U.S. Pat. No. 6,461,908 B1 proposes using a curved mask having a form identical to the form of the curved substrate. The curved mask is fabricated in a contact method.
The U.S. Pat. No. 5,257,139 discloses a purely reflective reduction objective for extreme ultraviolet radiation (EUV), wherein the object surface and/or the image surface are curved concavely with respect to the projection objective.
In order to be able to produce ever finer structures, firstly attempts are being made to increase the image-side numerical aperture (NA) of the projection objectives further and further. Secondly, ever shorter wavelengths are being used, in particular UV light having wavelengths of less than 260 nm, for example 248 nm, 193 nm or 157 nm. Reducing the feature sizes of the patterns present on the mask can also contribute to reducing the size of the structures produced on the image side. However, the fabrication of defect-free masks with fixedly predefined, extremely fine structures is complicated and expensive. Maskless technologies, such as are described by way of example in the article “Lithography: The road ahead” by D. A. Markle in: Solid State Technology, February 1999, p. 84 et seq., are therefore being developed.
The term “maskless lithography”, as is used in this application, relates generally to lithography techniques which do not use masks with fixedly predefined patterns, or do not exclusively use such masks.
One known, “maskless” lithography technique involves using reflective reticles (reflection masks) with individually drivable mirror regions. The pattern of the mask can thus be altered by suitable driving of the mask. The patterns to be imaged may be produced for example with the aid of deformable or movable micromirrors, as is shown in the U.S. Pat. No. 6,238,852 B1. The publication “Digital Micromirror Array for Projection TV” by M. A. Mignard, Solid State Technology, July 1994, pp. 63-68 discloses drivable micromirror arrays. The use of such planar mirror arrangements with an array of individually drivable individual mirrors as object to be imaged in projection exposure apparatuses is the subject matter of the patent specifications U.S. Pat. No. 5,523,193, U.S. Pat. No. 5,691,541, U.S. Pat. No. 6,060,224, U.S. Pat. No. 6,312,134 and U.S. Pat. No. 5,870,176. The projection objectives shown in the exemplary embodiments described therein are only represented schematically.
The applicant's international patent application WO 03/016977 A2 shows various projection exposure apparatuses of purely reflective construction which are suitable for projection exposure in the extreme ultraviolet range (EUV) and which use reflective reticles in the form of drivable micromirror arrays. Both the illumination system, by means of which the drivable reflective mask is illuminated obliquely, and the projection objective, which produces a highly demagnified image of the pattern formed by the reflection mask on the substrate to be exposed, have a center obscuration on account of the use of centrally perforated concave mirrors. An abaxial object field is illuminated during the oblique illumination of the planar reticle. Image-side numerical apertures of up to NA=0.6 are achieved given demagnifying imaging scales of up to 1:100.
The applicant's German patent application DE 100 05 189 A1 (corresponding to U.S. Pat. No. 6,596,718 B1) discloses projection exposure apparatuses having purely refractive or catadioptric projection objectives which use a planar reflective reticle with a fixedly predefined pattern. A beam splitter cube is provided in the reduction objectives, for which typical reduction scales of 1:4 and typical NA values of up to NA>0.5 are specified, said beam splitter cube serving for superposing the illumination beam path of the illumination system and the imaging beam path of the projection objective. The illumination light radiated laterally into the projection objective impinges centrally on the reflective reticle after reflection at the beam splitter surface. Optical elements that are utilized both in the illumination beam path and in the imaging beam path are situated between the beam splitter cube and the reflective reticle.
Laterally coupling illumination radiation into the imaging beam path of an optical imaging system with the aid of a beam splitter is known from the area of ultraviolet microscopy. The patent specification U.S. Pat. No. 5,999,310 shows an example of a UV broadband microscope with object-side NA=0.9 and variable magnification up to 100:1.
The U.S. Pat. No. 6,439,726 shows a projection objective—intended for video projection—with an integrated illumination system, when the optical axis of the illumination system is at a certain angle with respect to the optical axis of the projection objective. Inclined coupling of illumination light into a projection system is also disclosed in U.S. Pat. No. 4,969,730.
The terms “refractive projection objective” and “purely refractive projection objective” are used in this application interchangeably and in accordance with their ordinary and accustomed meaning in the art, to describe projection objectives whose imaging elements are all lenses.